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Iraqi soldiers stand along a road close to their vehicle in the area of Ein Tamer which leads west out of the central Iraqi Shiite Muslim shrine city of Karbala towards the mainly Sunni Muslim city of Fallujah. The U.S. has stepped up military aid to the restive country. (AFP)

The United States maintains a strong relationship and commitment with and to the government of Iraq," White House spokesman Jay Carney said, noting Washington has been in contact with Baghdad over Iraq's efforts to defeat Al Qaeda-linked groups that have been taking over parts of Fallujah and Ramadi.

"We have -- as Secretary [of State John] Kerry said -- made a significant commitment to helping the Iraqi government in dealing with that situation," Carney said. "And what Secretary Kerry's point also was -- and I think this was a broader point about conflicts in the region -- is that this is something for the Iraqis to take the lead on and handle themselves."

Iraqi forces were preparing to retake the city of more than 325,000 people while hundreds fled to escape shelling and air strikes by the government, observers said.

Mowaffak al-Rubaie, a former Iraqi national security adviser, has told the BBC that defending all of Iraq from attacks by Al Qaeda-linked militants is impossible.

"This can happen and probably will happen in future because you can't put millions and millions and millions of Iraqis under arms and protect every street all over the country," Rubaie said. "These guys [the militants], they are highly motivated, they are very well-trained, they can do this because they are brainwashed, their understanding of Islam is highly selective. They are brutal. They are ruthless. They are after anarchy."

Fighting also was reported in Ramadi, which like Fallujah is in Anbar province. Parts of Ramadi, the provincial capital, were controlled by militants.